Format insensitive and bit rate independent optical preprocessor

ABSTRACT

A method and circuit are presented for an all-optical format independent preprocessor that processes an arbitrary optical input signal by converting a NRZ signal to a PRZ signal, or if the input optical signal is RZ, by merely amplifying it. The method involves subtracting a delayed copy of the signal from the original, thereby effectively doubling its frequency, and inserting a pulse at each transition of the original signal, whether rising or falling. In a preferred embodiment this stage is implemented via an integrated SOA in each arm of an asymmetric interferometric device. The asymmetry consists of a delay element in one arm. In a preferred embodiment the entire device is fabricated on a semiconductor substrate, allowing for compactness as well as minimization of interconnectivity losses and overall power consumption. The output of the preprocessor, having a significant frequency component at its original clock rate, can then be fed to a clock recovery stage for all-optical clock recovery.

TECHNICAL FIELD

[0001] This invention relates to optical communications, and inparticular to processing of arbitrary optical signals for inputting toall-optical clock recovery systems.

BACKGROUND OF THE INVENTION

[0002] Optical fiber networks, such as SONET, are in widespread use dueto their ability to support high bandwidth connections. The bandwidth ofoptical fibers runs into gigabits and even terabits. Optical links canthus carry hundreds of thousands of communications channels multiplexedtogether.

[0003] As in all modern data networks, information is encoded as digitaldigits of “ones” or “zeros.” There are various formats used to encodethese digital bits as optical signals, depending on what type of pulsein the optical signal represents a “one” digit. Two of the most commonformats are Return to Zero (RZ) and Non Return to Zero (NRZ). In theformer, the second half of each bit is a zero, represented by noamplitude in the optical signal. Thus, each bit, no matter what itsvalue, has the zero amplitude level for its second half (in the timedomain, obviously). The latter format, NRZ, does not return to a zeroamplitude level each bit. Thus a sequence of 100 “one” bits would berepresented by holding the optical signal amplitude high for the timespan of 100 bits. Only when the bit is actually a zero does the signalamplitude go low.

[0004] For obvious reasons, the non-return to zero format uses bandwidthmore efficiently. The non-return to zero format is much more popular,and modem data networks tend to use a non-return to zero, or NRZ format.

[0005] The problem with NRZ format is that unless you know the inherentclock, it is very difficult to determine what the clock rate is of agiven NRZ signal. This is because if you have a string of high, or “one”bits, the NRZ signal simply stays high; if you have a sequence of lowbits, it simply stays low, there being no regular transitions. Thus, asan example, in NRZ format, three high bits followed by three low bitsfollowed by three high bits followed by another three low bits could beread as either 111000111000, or as 1010, with a clock speed one third asfast as the first alternative.

[0006] Whatever method is devised to process an incoming signal so thatit can be submitted to clock recovery analysis in the optical domain,that method must be able to preprocess the incoming optical signalsregardless of not only what format (RZ or NRZ) they happen to be in, butit also must be insensitive to the bit rate they happen to be in. As isknown, there are varying nominal bit rates supported in a network, suchas, for example, 10 GHZ, 20 GHZ, 30 GHZ and 40 GHZ, as well as variousmodifications to same resulting in various actual bit rates. This is dueto error correcting codes and similar format specific modifications tothe bit rate which use extra, non-data, bits for various managementfunctions.

[0007] As optical networks become increasingly transparent, there is aneed to recover the line rate in the form of a clock signal withoutresulting to any optical to electrical and back to optical conversion ofthe signal as is commonly done in the art. As networks tend towardsoptical transparency, the nodal devices in the optical network must workwith all supported line rates, independent of their format. One of thefundamental functions of these devices will be the capability to extractthe clock from the signal in the optical domain. This depends on thesignal's RF spectrum.

[0008] The RF spectrum of an RZ signal reveals a strong spectralcomponent at the line rate. Consequently, the incoming RZ signal can beused directly to extract the clock signal. All that needs to be donewith the incoming RZ signal is to amplify it. In the case of the NRZsignal format, the RF spectrum reveals no spectral component at the linerate, as described above. Thus, the RF spectrum of an ideal NRZ signallooks like a sinc function with the first zero at the line rate.Therefore, the fundamental problem of all optical clock recovery fromNRZ signals is the generation in the signal of an RE spectral componentat the line rate. Correlatively, the fundamental problem of all opticalclock recovery from an arbitrary signal is the simple amplification ofan RZ signal, and the conversion of an NRZ signal to one with asignificant spectral component at the line rate.

[0009] The output of such processing can then be fed to clock recoverysystems, which generally require an input signal with significantspectral component at the clock rate.

[0010] What is needed, therefore, to facilitate the next generation oftransparent, all optical data networks, is a preprocessor which can takea given arbitrary optical signal, pass the signal with amplification ifit is in RZ or other format with a large spectral component at the linerate, and take an incoming signal which does not have the large spectralcomponent at the line rate and process it so that it does. All theprocessing that is to be done on the incoming arbitrary signal must onlyoccur in the optical domain, so that optical-electrical-opticalconversion, which is both costly and adds complexity, is not required.

SUMMARY OF THE INVENTION

[0011] A method and circuit are disclosed for the preprocessing of anarbitrary optical data signal for later use by a clock recovery system.

[0012] The method passes RZ signals with amplification, and convertsincoming NRZ signals to Pseudo Return to Zero, or PRZ signals. PRZsignals have a strong spectral component at the line rate, and can beused by all optical clock recovery systems to lock onto that rate, thusgenerating an optical clock signal.

[0013] In a preferred embodiment, the method is implemented via aSemiconductor Optical Amplifier—Asymmetric Mach-Zehnder Interferometer,or SOA-AMZI, preprocessor, which, by controlling the SOAs in each arm,passes RZ signals with amplification, and converts NRZ signals to PRZtype signal, which has the requisite significant spectral component atthe inherent clock rate.

[0014] The method is bit rate independent.

[0015] In an alternative embodiment, the preprocessor is implementedusing a multimode coupler interferometer (MMCI). An exemplary SOA devicefor use in implementing the various circuits is also disclosed. Usingsuch a SOA device, the entire preprocessor can be integrated on a singlechip, thus facilitating all-optical integrated circuit network nodalprocessing functionalities.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 depicts a circuit implementing the method of the presentinvention;

[0017]FIG. 2 illustrates the method of NRZ to PRZ conversion;

[0018]FIG. 3 depicts a preferred embodiment of the circuit of FIG. 1;and

[0019]FIG. 4 depicts an exemplary semiconductor optical amplifier deviceused according to the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The above described and other problems in the prior art aresolved in accordance with the method, apparatus, circuit and devices ofthe present invention, as will now be described.

[0021] Most, if not all, optical networks currently operating transmitsome or all data as NRZ signals. In the case of the NRZ signal format,the RF spectrum reveals no spectral component at the line rate. This isa simple consequence of the format. The RF spectrum of an ideal NRZsignal looks like the mathematical sinc function with the first zero atthe line rate.

[0022] On the other hand, the RF spectrum of an RZ signal reveals astrong spectral component at the line rate. Consequently, an incoming RZsignal can be operated upon directly to extract the clock signal.

[0023] The fundamental problem of all-optical clock recovery from anarbitrary incoming optical signal is thus the passing of an RZ signalwithout attenuation, and the generation of a RF spectral component atthe line rate for a NRZ signal. For an NRZ signal of unknown bit rateand format, an NRZ/PRZ converter is used to generate this latterspectral component by converting the incoming NRZ into a pseudo returnto zero, or PRZ signal.

[0024] Although for illustrative purposes NRZ to PRZ conversion has beendescribed, in general any bit representation schema that does notcontain a significant RF spectral component at the line rate—i.e. wherethe recipient needs to know a priori what the line rate is so as todelineate each bit from its neighbors—can be converted to one that hassuch a component by the method of the present invention. The methodsimply exploits the fact that over the long run the transitions fromones to zeros are numerous enough to allow the creation of a signal withsignificant clock rate frequency component.

[0025] Once the incoming signal has a significant spectral component atthe line rate, optical oscillations, for example, can be triggered, byan optical clock recovery system, to obtain a pure line rate opticalclock signal.

[0026]FIG. 1 depicts an optical circuit implementing the method of thepresent invention. It consists of an input signal 100, which splits intotwo paths at point 130 via an interferomemter. In a preferred embodimentthe 50/50 splitter will send one half of the power to each inteferometerarm, although other ratios are possible and advantageous in certaincontexts, and thus contemplated within the method of the invention.

[0027] The two signals pass through the arms of the interferometer,which are not identical. The upper arm has a phase delay element 101,which can be implemented via a lengthening of the waveguide 131, or viaa specialized piezoelectric device. The interferometer is set fordestructive interference, and recombined at coupler 140. Thus the output120 of this circuit will have a pulse wherever a transition, either lowto high or high to low, occurred in the original signal. Each arm has anoptical amplifier in the signal path as well, for reasons to bediscussed after the illustration of the method of the NRZ to PRZconversion.

[0028] Thus, the interference of a high bit with its path delayed andphase inverted copy, generates an RZ-like bit at both the leading andfalling edges of the original high bit. This latter signal, with a bitrate effectively double that of the original NRZ bit rate, is the PRZoutput signal 120.

[0029] This effective doubling of the bit rate leads to the generationof a large component of the line rate frequency in the RF spectrum ofthe output signal 120 of the AMZI. Generally, unless the input signal isexceptionally aberrant, this line rate frequency will be the far andaway dominant frequency in the spectrum. Since the preprocessor does notneed to know the actual bit rate or format of the input data, it is datarate and format insensitive.

[0030]FIG. 2 illustrates the above described method. Signal 201 is thesignal traveling through the lower arm of the interferometer. It has thesame form as the NRZ input signal, less the power fraction lost at thesplitter 130. Signal 202 is the delayed signal generated in the upperarm of the interferometer, delayed approximately 25% of the NRZ bitwidth at a data rate of 10 Gbits/sec. This delay will vary for higherbit-rate signals. Thus when signals 210 and 202 are destructivelycombined, the output signal 203 has an appropriate pulse width. Therequired power of the pulses (as determined by the clock recovery systemto which this output signal 203 is input) is adjusted via the gaincontrol of the SOAs. In the example shown in FIG. 2, the output pulsewidth is approximately 25% of the NRZ pulse width (thus roughly equal tothe percentage delay) and an output pulse is generated wherever theoriginal signal transitioned.

[0031] In order to achieve true bit rate independence, the preprocessormust be able to handle any of the bit rates used in the network. Say,for example, this includes a bit rate spectrum of 10 Gbits/second to 40Gbits/second. Since the interferometer of an optical circuit such asdepicted in FIG. 1 cannot have the length of any of its arms dynamicallylengthened or shortened, nor can a piezoelectric delay element bedynamically controlled to vary the delay, the delay is fixed, and mustsupport all bit rates. Thus, one generally designs the circuit of FIG. 1to have a delay long enough to cause a substantial overlap at allsupported bit rates, given the fact that a fixed differental waveguidelength between the two arms of the interferometer produces a differentoutput pulse width at different bit rates.

[0032] One possible method of doing this is to set the overlap betweenthe original signal and the delayed version (determined by the delay ofthe upper AMZI arm relative to the lower AMZI arm) at the lowestsupported bit rate high enough, such that at the highest supported bitrate the output PRZ pulse widths are still wide enough, given the signalpower amplification possible via the SOAs, to drive the clock recoverysystem. (The absolute theoretical maximum delay being some delay closeto, but distinguishably less than, 100% of the NRZ pulse width, thus thefixed delay cannot be so large so as to be greater than 100% at thehighest supported bit rate). The longer the AMZI upper arm delay, thewider the pulses in the PRZ output, as is evident from FIG. 2. However,as the bit rates rise, the overlap, and thus the pulse width of the PRZsignal at that bit rate, will be substantially less than that for thelowest bit rate (since the delay time is absolute, but the pulse widthsare shorter in the time domain as the bit rate increases), and will thusrequire more amplification from the SOAs.

[0033] It is understood that different patterns in the NRZ input signalexperienced in any given network at any particular time may result inoutput signal variation; ongoing monitoring of the quality of the outputsignal 203 in terms of its ability to properly drive a given clockrecovery system may be required, and use of an AMZI with a slightlydifferent fixed upper arm delay may be useful. Where the spectrum ofsupported bit rates is very wide, or significant aberrant input signals,for whatever reason, are received, the method of the invention may beextended to an array of AMZIs. In such a topology an optical switchingmechanism would be utilized to switch the optical signal input pathbetween two or more AMZIs in the array, each having a slightly differentupper arm fixed delay. The switch would be under control of a sensingdevice, which would categorize an input signal as being in a particularsub band (determined by bit rate and/or other factors) most optimallyserviced by an AMZI with a particular fixed delay. Alternatively, anoptical device with a variable delay element, as may become available inthe art, could be used, its delay being controlled as a function of theinput signal.

[0034] As an illustrative example, for a 10 Gbit/s NRZ original signal,the bit width is 1/(10*10⁹) sec, or 100 picoseconds. Assuming the samenetwork also supports 40 GHz signals, the NRZ pulse width at that 40Gbit/s rate is now only 25 picoseconds. Setting a 50% delay at 40Gbit/second, or a fixed delay of 12.5 picoseconds, results in only a12.5% pulse width overlap at 10 Gbit/second. Alternatively, setting afixed delay in the AMZI of 20 picoseconds results in a 20% overlap at 10Gbit/s and a thus a 20 picosecond PRZ output pulse width. However, thesame delay at 40 GHz is some 80% of the pulse width, making thickerpulses in the PRZ signal at 40 GHz. If smaller overlaps are used, thepulse width power can be amplified by increasing the gain of the SOAs.

[0035] Thus, the use of dynamically controllable SOAs facilitates thebit rate insensitivity of the device notwithstanding a fixed delayelement in the interferometer. The use of the SOA-AMZI also allows theinput power required by subsequent devices in the signal path, such asthe clock recovery system to be quite nominal; thus signalpre-amplification concerns are diminished or avoided.

[0036] Multimode Interference (MMI) couplers with a 50:50 splittingratio (commonly known as 3 dB couplers) make up the couplers of thedevice 310 and 311, respectively.

[0037] The SOAs are dynamically controlled via feedback from the clockrecovery system to increase gain as necessary for lower bit rate signalsso as to keep the optical clock signal robust. Such feedback mechanismsare generally known in the art, and not further described herein.Additionally, if the incoming signal is sensed to be in RZ format, thenthe gain of the upper arm SOA is set to zero, and that of the lower armto double its normal gain. Thus, in such case, the RZ signal is simplypassed and amplified. Sensing of RZ vs. NRZ formats is generally knownin the art, and can be via a priori communication from a user in apacket header or frame header, or can be gleaned from an arbitrarysignal by means of specialized circuitry.

[0038] The method of the invention can be implemented using eitherdiscrete components, or, in a preferred embodiment, as an integrateddevice in InP-based semiconductors. The latter embodiment will next bedescribed with reference to FIGS. 3 and 4

[0039]FIG. 3 is the preferred embodiment integrated circuit version ofthe circuit of FIG. 1. The delay is achieved by lengthening thewaveguide in the upper arm 306 of the interferometer. As before, the NRZinput signal is split at coupler 310 into two parts. One travels on thelower arm, and is amplified. The other travels on the upper arm and isamplified, as well as delayed by the longer path length. The signals arerecoupled at the coupler 311, and the destructive interference resultsin the PRZ output signal 320.

[0040]FIG. 4 depicts a cross section of an exemplary integrated circuitSOA. With reference to FIG. 1, FIG. 4 depicts a cross section of any ofthe depicted SOAs taken perpendicular to the direction of optical signalflow in the interferometer arms. Numerous devices of the type depictedin FIG. 4 can easily be integrated with the interferometers of thepreprocessor, so that the entire circuit can be fabricated on one IC.The device consists of a buried sandwich structure 450 with an activeStrained Multiple Quantum Well region 411 sandwiched between twowaveguide layers 410 and 412 made of InGaAsP. The sandwich structuredoes not extend laterally along the width of the device, but rather isalso surrounded on each side by the InP region 404 in which it isburied.

[0041] The active Strained MQW layer is used to insure a constant gainand phase characteristic for the SOA, independent of the polarization ofthe input signal polarization. The SMQW layer is made up of pairs ofInGaAsP and InGaAs layers, one disposed on top of the other such thatthere is strain between layer interfaces, as is known in the art. In apreferred embodiment, there are three such pairs, for a total of sixlayers. The active region/waveguide sandwich structure 450 is buried inan undoped InP layer 404, and is laterally disposed above an undoped InPlayer 403. This latter layer 403 is laterally disposed above an n-typeInP layer 402 which is grown on top of a substantially doped n-type InPsubstrate. The substrate layer 401 has, in a preferred embodiment, adoping of 4−6×10¹⁸/cm⁻³. The doping of the grown layer 402 is preciselycontrolled, and in a preferred embodiment is on the order of5×10¹⁸/cm⁻³. On top of the buried active region/waveguide sandwichstructure 450 and the undoped InP layer covering it 404 is a laterallydisposed p-type InP region 421. In a preferred embodiment this regionwill have a doping of 5×10¹⁷/cm⁻³. On top of the p-type InP region 421is a highly doped p+-type InGaAs layer. In a preferred embodiment thislatter region will have a doping of 1×10¹⁹/cm⁻³. The p-type layers 420and 421, respectively, have a width equal to that of the activeregion/waveguide sandwich structure, as shown in FIG. 4.

[0042] As described above, the optical signal path is perpendicular toand heading into the plane of FIG. 4.

[0043] As discussed above, utilizing the SOA described above, theoptical preprocessor can be integrated in one circuit. An exemplarymethod of effecting this integration is next described.

[0044] After an epiwafer is grown with the waveguides and the SOA activeregion, the wafer is patterned to delineate the SOAs, and the AMZI. In apreferred embodiment the path length difference between the two arms ofthe AMZI is approximately 1 mm.

[0045] The top cladding layer (undoped InP), the p-type InP layers andthe contact layer are then regrown on the patterned substrate. This stepis then followed by photolithography for top-contact metallization. Thedevice is then cleaved and packaged.

[0046] The disclosed preprocessor can serve as a pre stage of an AllOptical Clock Recovery (“AOCR”) scheme. Thus the entire circuit canitself be integrated on a larger chip with such clock recovery, andother (such as all optical regeneration, reshaping and retiming)integrated functionalities.

[0047] While the above describes the preferred embodiments of theinvention, various modifications or additions will be apparent to thoseof skill in the art. Such modifications and additions are intended to becovered by the following claims.

What is claimed:
 1. A method of processing an arbitrary optical datasignal, comprising: amplifying the signal if it has a high spectralcomponent at its original clock frequency; and transforming the signalto cause it to have a high spectral component at its original clockfrequency if it does not.
 2. The method of claim 1, where the method iscarried out completely in the optical domain.
 3. The method of claim 2,where the method is carried out with no loss of information;
 4. Themethod of claim 2, where the signal transformation is effected bydestructively interfering a delayed copy of the optical signal withitself.
 5. The method of claim 2, where the pulses in the output signalhave widths proportional to the bit rate.
 6. A method, comprising:amplifying an optical signal in NRZ format; and processing the signal soas to cause it to have a high spectral component at its original clockfrequency.
 7. The method of claim 6, where said processing includesdestructively interfering the signal with a delayed version of itself.8. The method of claim 7, where the interference is implemented in aMach-Zchnder interferometer.
 9. The method of claim 7, where theinterference is implemented in an MMCI interferometer.
 10. The method ofclaim 8, where the amplification is done via an SOA in the signal pathof each arm of the interferometer.
 11. The method of claim 9, where theamplification is done via an SOA in the signal path of each arm of theinterferometer.
 12. An optical circuit, comprising: an interferometricdevice; optical amplifiers in each arm of the interferometric device;and a delay in one of the arms.
 13. The circuit of claim 12, where theoptical amplifiers are SOAs.
 14. The circuit of claim 13, where thedelay is effected with a piezoelectric device.
 15. The circuit of claim13, where the delay is effected by lengthening one of the interferometerarms.
 16. The circuit of claim 12, where the optical amplifiers aredynamically controllable.
 16. The circuit of claim 13, where the opticalamplifiers are dynamically controllable.
 17. The circuit of claim 12,where the circuit takes as input an optical signal of arbitrary bit rateand format.
 18. The circuit of claim 13, where the circuit takes asinput an optical signal of arbitrary bit rate and format.
 19. Thecircuit of claim 16, where the circuit takes as input an optical signalof arbitrary bit rate and format.
 20. A semiconductor device comprising:an InP substrate of a first doping type; a second InP layer of the firstdoping type disposed upon it; a third InP layer not doped disposed uponsaid second layer; a first InGaAsP waveguide region laterally disposedon top of said third InP layer, whose width is less than that of thesubstrate, first and second InP layers; an active strained multiplequantum well (“SMQW”) region laterally disposed and centered on top ofsaid first waveguide region, having the same width as said firstwaveguide region; a second InGaAsP waveguide region laterally disposedon top of said SMQW layer, having the same width as said first waveguideregion and as said SMQW region; a fourth InP layer, undoped, disposedupon said second waveguide region, and extending downward, in thedirection of the substrate, along the sides of said active region andsaid first waveguide region, whose width is equal to that of thesubstrate, and the first and second InP layers; a first InP layer of asecond doping type, laterally disposed above said fourth InP layer,having the same width as said first waveguide region and as said SMQWregion; a seond InP layer of the second doping type, laterally disposedabove said first InP layer of the second doping type, having the samewidth as said first InP layer of the second doping type; a contact layerlaterally disposed above said second InP layer of the second dopingtype; and a metal electrode disposed above said contact layer.
 21. Anintegrated optical circuit comprising: the optical circuit of any ofclaims 12-19; and the semiconductor device of claim 20.